Automatic phase control circuit



Feb. 10, 1953 J. H. ROE 2,628,279

AUTOMATIC PHASE CONTROL CIRCUIT Filed March 3l, 1949 MQW y Patented Feb. l0, 1953 An'roMA'ric .PHASE CONTROL CIRCUIT John H. Roe, Collingswood, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application March 31, 1949, Serial No. 84,705

'l' Claims,

My invention relates toa phasing circuit and in ,particular to a circuit for maintaining constant phase relationships between a series of pulses and motor driven equipment.

When good average synchronism is required between operation vof a synchronous motor 'and electronic devices, it is sufficient that they be driven from a common electrical power supply. However, for obtaining exact phase synchronism, such a method is unsatisfactory if the equipments have diiierent inertias because a change in the magnitude or phase ofi-the power supply volta-ge is not followed at the same rate by, each piece of apparatus.

One obvious method by which this difficulty may be overcome is to use apparatus having the same inertia, but this presents diflicult design problems and limits the type of equipment that may be used in a given system. My invention on .the other hand produces good results when operated in conjunction -with equipments having different inertias.

Another method that has been employed with success is the loading of equipment having relatively small inertia when the power supply tends to increase its speed and removing the load when the .power supply tends to decrease its speed, but this necessitates maintaining an average load on the apparatus having the small inertia.

According to my invention the phase of one of the equipments being synchronized is altered by changing the conditions-of operation-in accord-ance with the changesfrequenoyor -amplitude of the power supply.4 -In this way equipment having diiierent inertiaswcannbe synchronized without loading when transients are present in the power supply.A l

However, it is not intended, formreasonsvthat will become apparent from a consideration of the detailed description below, that my invention be employed to maintain theaverage frequencies of varies apiparatusat a constant value, but rather that it be used .to maintain exact phase synchronism between various pieces of equipment during the. occurrence of transient phenomena existing in a common' power supply.

The problems discussed above are encountered in televising motion pictures as it is necessary to achieve exact phase synchronism between the. motion picture projector and the control or, sync pulse generator. In such equipment itis customary to project each frame of a motion pic# ture film onto the target of the television picki Cl.

up tube when the latter is not producing signals. More specifically, the picture is projected in a short pulse of light that normally occurs during vertical retrace time when the electronic beam is being moved back to the point where it starts scanning the target. If the light pulse from the picture projector, or any part of it, occurs during the scanning of the target in the pick-up tube, thatpart of the target scanned during the interval the picture is projected produces a much whiter background than is produced over the rest of the surface so that a horizontal white bar appears in the picture. Under some conditions of back-lighting in the pick-up tube, the bar can be black rather than white, but in either case the vertical position of the bar is determined by the amount of phase difference between the projector and the sync pulse generator as the latter determines when the scanning of the target begins. vIn most of the systems for televising motion pictures, the movie projector and the electronic pulse generator are driven by a common power supply and the sync pulse produced by the generator not only controls the starting point of the frame scanning cycle at the pick-up tube but is transmitted with the video intelligence so as to control the scanning of the receiver. Until recently, satisfactory phase synchronism has been obtained by employing the method discussed above in which the mechanical inertia of the motion picture projector is made equal to the inertia of the sync pulse generator. However, recently promulgatedstandards have increased and required inertia of the sync pulse generator to such a degree that matching the mechanical inertia of the motion picture projector with the sync generator presents a diiiicult design problem.

My invention operates so as to develop a direct current voltage having a magnitude proportional to the diiierence in phase between the movie projector and the sync pulse generator, and this voltage is applied, through suitable amplifiers, so as to control the field excitation of the synchronous motor that drives the projector. If the direct current field of a synchronous motor is under excited, the rotor lags the rotating magnetic iield produced by the stator and, conversely, if the di rect current field is over excited, the rotor leads the rotating magnetic field. Therefore, in as much as the inertia of the synchronous motor and the projector is less than that of the sync pulse generator, the current supplied to the eld by the phase comparator circuit should be less than the normal amount when the voltage or frequency of the common power supply changes in such a way as to cause the generator and projector to speed up, and greater than the normal amount when the power supply changes in such a way as to cause them to slow down.

By employing my invention, it is possible to achieve phase synchronism between the sync pulse generators and motion picture projectors having diierent inertias and thereby prevent white or dark bars from appearing inthe televised image.

Accordingly, it is an object of this invention to provide a circuit for properly phasing a syn chronous motor with a series of pulses.`

A further object of this invention is the automatic control of the phasing of a television motion picture projector and a synchronousfpulse generator.

It is a further object of this invention to provide rapid phase synchronizing between the pulse generator and the motion picture projector so as to eliminate distortion on the television screen.

Various other objects will become apparent from a detailed analysis f the drawing wherein- Figure l is a schematic diagram of the phase 'f control circuit;

Figure 2 shows by circuit diagram another form of this invention; and

Figures 3a and 3b show graphically the operation of my'invention.

InFigure i there is shown a circuit for improving the phasing of electrical equipment in which numeral 2 indicates a synchronous motor having stator windings, a rotor, t, and a direct current eld, B. y drive any type oi equipment indicated 'by rectangle il), butin this particular instancethe driven equipment is assumed to be a lmotionpicture projector'.` Also driven by the rotor is a pulse generator I2, thcoutput electricalwave form of whichV is illustrated 'in Figure 3a. The current in the direct current field 8 is controlled by direct current amplier it, and it is to beunderstood that several of these could be used in parallel in orderto increase the current capacity. rlhe grid V5 of tube it', is connected directly to the plate I3 oi tube 2B. The cathode 22 Vof the latter tube isconnected through a 4source of bias potential 2d to the pulse generator l2, the Vgrid 26 being driven by the ypotential across' condenser 26 which is connected between the grid and the junctionv between the pulse generator Vand source of bias potential. *Although this' junction is grounded, it'is readily understoodv that ground could be located elsewhere. A normally open z electromagnetic` relay 3i) remains closed inthe presence of YsynchronousV pulses that 'are provid# ed from anyrsource 32;`

The amount'of' current iiowing in the direct current neldc 'is dependent on the chargemaintained on condenser 28; and if the relay 'is open,=the pulses shown inf Figure 3a, that are generated by the" pulse generator I2, are not able to charge the condenser23.l However, when the pulses, rsuch-as Vshownfin 3b,'close'rela`y 313 during the timefthat the pulses showninFig'ure 3a areapplied, the condenser 23 will'be charged and the current in thedirect veld` winding 8 is changed accordinglyx Whereas' thisdevice is satisfactoryformany purposes;` it is limited to *f operation vin the low frequency range because the relays now-available are unable to follo'whigh frequencyimpulses.

'In order to V'overcome this problem the rectifier bridge circuit shown in Figure 2Y is connected, at

This motor may be used to points A and B as shown. The rectier bridge circuit of Figure 2 supplants the circuit of Figure 1 to the left of point A and point B and is an alternative therefor. One side of the bridge is composed of cppositely connected diodes 34 and 36, the plate 38 of diode 34 being connected to point A and the plate 4G of diode E' being connected to point B. The other side or" the 4bridge is composed of oppositely connected diodes t2 and 134, the former having its cathode i5 connected to point A and latter having its cathode d8 connected to point B. Between the diagonal points, 50 and 52, of the bridge there is connected a parallel R. C. network comprising resistor Stand condenser 56 having a long time constant with respect to the duration or the longer of the pulses employed. In series with this R. C. circuit is a transformer secondary 58, the corresponding transformer primary 52 being connected to a source of external synchronous pulses 32.

Assuming that the output wave form of pulse generator I2 is that which is shown in Figure 3a, it can be seen that condenser 28 will be charged via the following path: diode'36, junction 50, junction 52 and diode 42, in such manner that the upper plate of the condenser will be positive and the electrons will be deposited on the lower plate. The condenser 56,"however, will be charged so that the right hand `plate is negative and the left hand plate is positive. Between the pulses shown in'Figure 3b the condenser discharges through resistor `54` so that thed'ighthand end of the resistor is negative and the left hand end is positive thereby keeping the diodes nonconducting between pulses. Now if the'pulses Yshown in Figure 3b are of'such polarity as to induce in secondary 58 'a voltage that is opposed to that provided bythe RIC. network, then it is seen that condenser 23Y can be'chargedby the signals from generator l2, and if'th'ese pulses are of short duration with respect to the signals of generator l2, it can be seen that the amount of charge stored on condenser 28 will be determined by the relative phase between them. VFor example, the charge path is operative during the presence of the square pulses shown in Figure 3h and since the amount f charge stored `on condenser 23 is'dependent on the magnitude of the signal from generator I2 a' greaterv charge is'd'eveloped when the two waves are nearly in`phase as shown at 53, than when they are out of phase as shown at 55.

/It is necessary when unilateral devices such as "diodes ar"`used to` provide a Vdischarge path forcondenser 23 'because vonce the condenser is charged by 'the'pulses of'Figures 3a and 3b to a given-value an increase-inthe phase difference betweenthem froma phase such as shown at 5l to a phase such as shownfatv 53 would not reduce the condenser "charge, and the current through the direct'A current 'field controlled thereby would remain the saine. Condenser 28Y is, therefore, disclnrrged via the following path: B, diode te, junction'52, junction a'nddiode, and A, because thei voltage across condenser 28j is greater than the voltage of signal introduced by generator i2 and of such polarity'that the charge path is nonconductive andthe discharge path conductive. Y

The* operation rof the circuit is therefore seen to bea's vfollows'. VVN OYII.ally when the synchronous motor 2 is rotatingat constant` speed the relationshp' betweentheoutput signals from gen'- erato'r l2 and synchijonous pulse generator 32 Ill are as shown at 5| of Figures 3c and 3b respectively. Now if the phase of the power supply applied to stator 4 of the synchronous motor should lag, or in other words if the frequency should momentarily decrease, the signal developed in generator i2 would be advanced with respect to the power supply voltage and the rotating magnetic field because of the inertia of the shaft 5 and the driven equipment it, but the synchronous pulses produced by generator 32 would lead by a greater amount because as stated above its inertia is greater than the mechanical inertia of the driven equipment l0 and naturally of the whole rotor 6. This means that the pulse shown in Figure 3b would advance in phase with respect to the pulse shown in Figure 3a as shown at 53 and accordingly, a maximum amount of voltage is developed across condenser 28, thus decreasing the amount of current in the held winding 8 and advancing the space phase of the `rotor t with respect to the magnetic eld produced by stator d. Therefore, the rotor G and driven equipment l0 are returned nearly to the original phase relationship with generator 32 which it is remembered was less aifected by the phase shift of the power supply frequency. The pulses from generator I2 and synchronous pulse generator 32 are thereby returned nearly to the initial point 5i. It is obvious that an advance in phase of the power supply system would retard the rotor B and driven equipment IE! with respect to the stator iield and advance it with respect to the sync pulse generator until the pulses are nearly in. the phase shown at 5 i.

In order that the above system have its maximum effect it is desirable that the wave forms shown in Figure 3a be fairly steep in order that a maximum change in charge on condenser 26 can be produced with minimum change in relative phase between the wave forms. If the synchronous motor should be constantly losing or gaining in phase, it can be seen that this system cannot correct the change because once the pulse shown in Figure 3b occurs either to the right o' the peak or the left of the zero point of the wave shown in Figure 3a, the charge stored on condenser 23 produces the opposite effect desired. Therefore, this system is designed to work in cooperation with a frequency control system and to provide very fine control capable of controlling phase relationships within a very few degrees.

Whereas this phase control circuit has been described in combination with a synchronous motor, it is readily adaptable to other types of driving equipment. However, because the limits of control described above are about the same as that of a synchronous motor, they are peculiarly well adapted to work together.

Having thus described the inventifmy what is claimed is:

l. A phase shift system comprising a source of a irst group of pulses, a synchronous motor, a direct current iield winding associated with said synchronous motor, means for supplying direct current to said winding, means for generating another group of pulses in constant phase relaship with the rotation of said synchronous motor, a means for developing an output control voltage having a magnitude determined by the phase relationship between said two groups of pulses, and a means for utilizing said control voltage to vary the current in said winding in such man- .er as to maintain the phase between the two groups of pulses at a constant value.

2. A phase synchronizing sys em comprising a synchronous motor, a direct current eld winding associated with said motor, a source for supplying said eld winding with direct current, an energy storage device, a signal generator driven by said motoi, and a switch connected in series, a source of synchronizing pulses, means for placing said switch in conducting condition during said pulses so that the storage means is charged or discharged with energy depending on the relative potential levels of the signal generated and the existing energy potential on said storage means during said pulse interval, and means for controlling said source of direct current in accordance with the quantity of energy stored in said storage means.

3. A motor phase control system comprising a synchronous motor, a direct current field winding operatively associated with said motor, a motor shaft, a signal generator coupled to said shaft, a rst pair of diodes connected in series, a second pair of diodes connected in series, parallel with said rst pair, a transformer having a primary and a secondary, said secondary being connected in parallel with said diodes, connecting means between the output of said generator and the junction between the first pair of diodes, a source of signals with which the motor is to be synchronized, said source being connected to said primary, an output circuit, said circuit being connected between the junction intermediate the second pair of diodes and said eld winding.

4. A phase synchronizing system comprising a prime mover, a signal generator driven by said prime mover, an energy storage element, and a switch connected in series, said switch comprising a bridge network, one side of said bridge having a rst pair of diodes connected in series opposition, the other side of said bridge having a pair of diodes connected in series opposition and in the opposite sense to said rst pair, one junction of said arms being connected to said storage means, the other junction of said arms connected to said signal generator, a parallel resistor capacitor network, a transformer secondary, said network and said secondary being connected in series between the junction points between each of said pairs of diodes, a primary in inductive relationship with said secondary, a source of pulses connected in circuit relationship with said primary, and a means for controlling the phase of said prime mover in accordance with the energy level in said storage means connected in operative relationship between said prime mover and said storage means.

5. A circuit for synchronizing a motion picture projector used in a television transmitter with the 'sweep synchronized pulses controlling the sweep of said transmitter pickup tube comprising a synchronous motor, a direct current field winding associated with said motor, a source of direct current potential connected to said iielc winding, a motion picture projector driven by said motor, a signal generator driven by said motor, a condenser, and a switch, said generator, condenser, and switch being connected in series, said switch comprising a bridge circuit haring two arms connected in parallel, one having a pair of diodes connected in series opposition, the other having a pair lof diodes connected in series parallel and in the opposite sense to said first mentioned pair, one junction of said arms beingr connected to one plate lof said condenser, the other junction lof said arms being connected to said pulse generator, a parallel resistor capacitor combination, and a transformer secondary ccnnected in series between the midpoints of said arms, `a Iprimary connected in inductive relation'- ship with said secondary, said primary being in circuit with said source of sweep pulses, and means for controlling said direct current' source in accordance with the lcharge on said condenser.

6. In apparatus for maintaining a precise phase relationship lbetween the rotation oi a synchronous motor and pulses developed by an electronic circuit, the synchronous motor and the electronic circuit being operated by -a common power supply and having diierent amounts oi inertia, the combination of means for generating pulses havingr a constant phase relation to the rotation of said motor, a phase comparison circuit, said phase comparison circuit being coupled so as to receive the pulses supplied by said electronic circuit and said generating means and produce a control voltage indicative lof the phase relationship between said pulses, a `direct current field Winding operatively included in 'said synchronous motor, and means for controlling the current flowing therein in response to said control voltage. Y

7. Apparatus as described in claim 6 having a phase comparison device that is adapted to pass current only when the pulses' applied to it -coincide, a condenser connected in series with Ksaid phase comparison device so as to be lcharged or ldischarged only when said phase comparison device is capable of pass-ing current, said condenser 'being coupled so that the voltage across it is applied to the means for controlling the current on the D. C. eld of the synchronous motor.

JOHN H. ROE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,807,190 Bock et al. Nay 26, 1931 2,228,079 Gulliksen Jan. '7, 1941 2,399,421 Artzt Apr. 30, 1946 2,419,637 Gabriel et al. Apr. 29, 1947 2,428,946 Somers Oct. 14, 1947 2,443,195 Pensyl Jan. 15, 1948 2,474,829 Curtis July 5, 1949 2,474,886 Bovey July 5, 1949 

